Thanks, but I'm not sure what colors to select to make that Lab image mathematically correct. I'll play around, see what I can come up with (but then never be certain I did it right).
Interestingly, when looking for a ProPhoto color profile to install on my computer (what lead up to this whole test-chart thing, mentioned on this page
Understanding ProPhoto RGB), I ran across this add-on from MickySoft, a
color-profile control panel for free that you can install on XP, looks interesting.
time passes ...
Well, I finally got it to work. None of the methods you suggested worked. But a variation of one did. I made 2 L-gradients in the required directions in a Lab document, then saved them as Photoline documents (*.pld). Then using the Change Channels tool, I used that to move the files into the required channels.
Seems like quite the work-around for something that should work. (btw: PL gives a "major error" warning if you try to open a file in the Change Channels tool but don't actually select one to import into a channel).
After seeing what had to be done to get it to work, methinks that PL32 needs some work in this area.
Hmm... interesting. I accidently clicked on my Granger Chart that I made with PL32, while I had the flood-fill tool still active with a gray gradient. The tool's settings were set at Tolerance 100%, but it only filled in an area of color. I thought tolerance 100% meant that it would match ANY color or luminosity. Yet another area that PL32 needs fixing?
And one more thing that needs fixing. My "UNDO" sometimes doesn't undo to the beginning. It sets the first step as sometimes the 1st, 2nd, or 3rd step. Maybe this is a memory limitation or something. Working with these large 16 bit files might be showing me a limit I didn't know I had. (Duh, that was it, I had left PL32's default Undo set to 128MB.)
Oh, one more weirdness that I noticed while doing this. When flood-filling a gradient a second time, the gradient would be off-set to one side or the other, filling up only part of my image. I never could figure out what was causing that, no matter how much I tried to center the mouse before flood-filling, it would shift it off to the side and leave part of the image untouched.
(While testing all this, 6 deer were grazing right outside the house, and are now laying down next to the house while I type this, so I had to snap off some photos of them too.

)